We shall continue analyzing the detailed neuronal morphology and patterns of connectivity in an anatomically circumscribed region of the central nervous system, namely, that region in the spinal cord of the cat - the most medial portion of the seventh lumbar segment of the dorsal horn - in which physiological study has revealed the location of interneurons mediating the plantar cushion (PC) reflex. The neuroanatomy should complement the physiology in providing detailed evidence of the anatomical substrate for transmission of this reflex. Because the circuitry of only a small fraction of the spinal gray matter is primarily involved in PC reflex transmission, it is possible that this anatomical analysis can be more successful than attempts to analyze the circuitry of the dorsal horn as a whole. Various modifications of the horseradish peroxidase technique will be employed, including iontophoretic application within the spinal cord, diffuse filling of cut primary afferents, and intracellular injection. BIBLIOGRAPHIC REFERENCES: Proshansky, E. Selective staining of primary afferents to the spinal cord dorsal horn by anterograde movement of HRP. Anat. Rec., 1977 (abstract). In press. Proshansky, E. and Egger, M.D. Dendritic spread of dorsal horn neurons in cats. Experimental Brain Research, 1977, in press.